The present invention is in the technical field of inductively heating and stirring electrically conductive molten materials wherein the heating and stirring can be accomplished simultaneously.
It is well known in the art to melt an electrically conductive material, such as a metal, to heat the molten metal (or melt), and to hold the melt at a temperature by placing the metal in an induction furnace or holding crucible and magnetically coupling the metal to an ac magnetic field. The field is produced in one or more induction coils surrounding the crucible by the flow of ac current from a power source. To maintain sufficient electromagnetic stirring, the electrical frequency of the current is reduced as the furnace capacity increases and the applied ac induction power (and current) increases. For example, a furnace with a melt capacity of 35,000 pounds (16 tonnes) of iron has an optimal power supply frequency of approximately 150 Hz, whereas a furnace with a melt capacity of 5,000 pounds (2xc2xc tonnes) of steel has an ideal power supply frequency of approximately 600 Hz.
It is also well known that a melt subjected to an ac magnetic field will move when eddy currents generated in the melt by the applied field produce a flux field that opposes the applied magnetic field. Generally, fields produced by higher frequency currents will result in little stirring action and fields produced by lower frequency currents will result in preferred electromagnetic stirring motions with circular-like flow streams through the melt. Further the turbulence of the flow will increase as the magnitude of the applied field (supplied current) is increased.
For some melt compositions and applications, the pre-selected frequency of a single ac power supply may provide both heating and stirring actions that are sufficient for the process. In other applications, separate heat and stir frequencies may be used. There are numerous prior art approaches to applying ac power to a melt at two different frequencies to achieve the heating and stirring functions. Earlier approaches focused on using switching arrangements that alternatively isolated heating and melting power sources from the induction coil sections. Switching arrangements are disadvantageous in that they do not allow for simultaneous heating and stirring of the melt and require additional system components.
Later approaches focused on system topologies that simultaneously applied heating power (operating at a pre-selected heat frequency) and stirring power (operating at a pre-selected stir frequency). A significant technical problem to be overcome in these systems is adequate electrical isolation between the simultaneously connected heating and stirring ac power supplies. Failure to provide this isolation when electronic ac power sources are used can result in component malfunction or failure in a power supply that has its output connected to a second power supply operating at a different output voltage and/or frequency.
One solution to this technical problem is identified in U.S. Pat. No. 5,012,487, entitled Induction Melting (the 487 patent). FIG. 1 is a simplified schematic that represents the prior art teachings of the 487 patent. In FIG. 1 an electrostatically screened three-phase transformer 126, having primary windings 124 and secondary windings 128, is used to provide stirring power to three coil sections, 114a, 114b and 114c, that make up an induction coil for an induction melting vessel. Stirring power is provided from a 50 Hz, three-phase power source 120 (utility service power). The transformer also uses a tertiary three-phase winding 127 that feeds a three-phase delta-connected power factor correction arrangement (not show in the simplified schematic). Capacitors 138a, 138b and 138c are connected to the three coil sections as shown in FIG. 1. The high voltage single-phase output of the heating power source 136, operating in the frequency range of 150 Hz to 10 kHz, provides heating power to the coil sections through the capacitors. By selecting the impedance of the capacitors, the coil sections and the secondary of transformer, so that the resultant L-C series circuit is at resonance for the operating frequency of the heating power supply, heating power is transferred from the heating power supply to the coil sections. The 50 Hz stirring power source, operating at off-resonant frequency, is impeded from being applied to the input terminals of the heating power source 136 by the tuned series-resonant circuit. Conversely, heating power is blocked from the stirring power source since the secondary windings of transformer 126 are effectively in parallel at the operating frequency of the heating power source.
There are a few disadvantages to the circuit arrangements disclosed in the 487 patent. Power transformer 126 is an expensive component with voltage tap changers (not shown in the simplified schematic) and the tertiary winding as further described in the 487 patent. Further the operating frequency difference between the heat power source and the stir power source must exceed a certain range for the series resonant circuit to operate effectively. This is particularly problematic for large capacity induction melting vessels.
Therefore, there exists the need for apparatus for and method of simultaneously induction heating and stirring a melt from two separate power supplies, without the use of isolation transformers or switches, wherein the frequency of stir power supply (and induced stir field) is less than the frequency of the heat power supply (and induced heat field), particularly when the frequency of the heat power supply is close in frequency of the stir power supply.
In one aspect, the invention is apparatus for and method of simultaneous induction heating and stirring of an electrically conductive material in a vessel having at least one set of three interconnected induction coil sections disposed around the vessel. Inductive heating of the electrically conductive material is accomplished by applying single-phase ac power across the coil sections via one or more tuning capacitors and stirring of the electrically conductive material is accomplished by applying three-phase ac power to the coil sections via one or more inductors. The capacitive heating circuit and the coil sections operate at or near a first resonant point and the inductive stir circuit and the coil sections operate at or near a second resonant point to block power transfer between the sources of the single-phase and three-phase ac power.
These and other aspects of the invention are set forth in the specification and claims.